The present invention relates to providing telephone service via PBX to remote phones, and more specifically to providing direct public switched telephone network (PSTN) service thereto.
FIG. 1 illustrates a prior art system 10 in which plural remote telephones 12a, 12b, 12c behave as if they are directly connected to a PBX 14. A frame relay network 16 and its individual FRF.11 (Frame Relay Forum implementation agreement 11, Voice over Frame Relay) trunk connections 16a, 16b, 16c between the remote phones 12a, 12b, 12c and PBX 14 enable remote phone use, for example, when a user is at home, e.g. an employee is telecommuting, or at a branch office. Voice-equipped routers 18a, 18b on either end of network 16 perform call routing, as is known. PBX 14 will be understood typically also to have plural local telephones 12d, 12e, 12f directly connected thereto via a PBX station interface indicated as a dashed outline 20 (typically a part of PBX 14). Those of skill in the art will appreciate that interface 20 maintains status bits indicating the status, e.g. on-hook or off-hook, do-not-disturb, busy, etc., for each PBX-connected phone.
Voice over packet technology (e.g. FRF.11 or Voice Telephony over ATM (VToA) AAL2 (ATM Adaptation Layer 2) trunks) enable the connection of remote telephones to standard PBX station interfaces in support of telecommuters and small branch offices that desire connections to a main corporate PBX. These connections are often implemented with a simple fixed point-to-point topology, as described above by reference to FIG. 1, such that the on-hook or off-hook status of the phone is simply replicated at the PBX station interface. This enables standard PBX features like forwarding a call to voicemail upon a busy signal or a ring-no-answer condition to continue to function as if the phone were still directly attached to the PBX. One disadvantage of this approach is that the remote phone on the remote user's desk can place calls only via the main PBX. If the remote user wishes to place calls via a local PSTN connection, then the user requires a second phone. Also, in the branch office case, calls between users at the branch office are routed via the central PBX and consume data bandwidth on the voice over packet connection between the branch site and the main site.